Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 202

demand, when fewer conventional generators are online (Figure 5.34). Using
a simulation of the Ireland power system, during the summer minimum load, a
350 MW loss of generation occurs after 5 seconds, causing the frequency to fall.
Since the DFIG turbines do not provide an inertial response the frequency falls
more quickly and to a lower nadir, as compared with wind farms comprised entirely
of fixed-speed turbines. The reduced system inertia, however, does allow the sys-
tem frequency to recover faster.
With the expected expansion of wind farms sited offshore a further concern
arises, irrespective of the wind turbine technology. Early offshore wind farms were
located near to shore in shallow depths, with high voltage AC connection to the
mainland transmission network. The largest of these are Horns Rev (160 MW),
located in the North Sea, and Nysted (166 MW) in the Baltic Sea, both off Denmark.
More recent offshore wind farms, however, are much larger (250-1,000 MW) and
can be located further offshore in deeper waters. This presents a number of diffi-
culties. For AC transmission, losses increase significantly with distance. This can be
partially countered by increasing the operating voltage, but at the cost of larger and
more expensive transformers, cabling and switchgear. Increased cable lengths (and
hence capacitance) will also require greater use of reactive power compensation
(e.g. static VAr compensators), placed possibly at both ends of the cable. Beyond a
distance of 100 km, high voltage DC transmission becomes economically viable -
a horizon that is likely to move inshore with advances in technology. DC trans-
mission can reduce cabling requirements and power losses, while also offering
flexibility of both real and reactive power control (Kirby et al. , 2002). One dis-
advantage of this approach, however, is that irrespective of the wind turbine tech-
nology, the DC connection decouples the stored energy of the turbine rotor from the
electrical grid, i.e. an inertial response is not provided. It is likely that the system
operators will require these large wind farms to provide pseudo-inertia along the
lines of Figure 5.33, possibly utilising the energy stored in the DC link capacitor.
5.3.8 Distributed generation protection
A significant fraction of existing wind farm installations have been connected at
relatively low voltages to the distribution network. Such embedded generation is
assumed to be of small individual rating ( < 10 MW), scattered across a large geo-
graphical area, probably under independent control, and unlikely to be continually
operational. In addition to unit protection, against turbine overspeed, terminal
overvoltage, etc., the wind farm will be fitted with network protection, such that the
wind farm will be isolated from the power system under certain conditions (see
Section 4.9). For example, in the event of a network fault and the operation of line
protection, an embedded generator may become islanded from the main power
system (Figure 5.35). Although the wind farm could possibly supply some local
load, the voltage and frequency levels would be uncertain, potentially exceeding
legal and operational limits and leading to equipment damage. The generator may
also be unearthed, compromising normal protection devices, while synchronisation
procedures would be required later to integrate the islanded part of the network. It
is thus advisable that the wind farm disconnects itself in these circumstances. For
example, Engineering Recommendations G59/1 and G75 (Recommendations for